Adsorption of CO on Pd(100)

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 Å and a C–O bond length of 1.15±0.1 Å, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short-range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with coverage is described in terms of a second-order precursor state model with repulsive interactions. The pre-exponential factor for desorption (3×1016 sec−1) varies only little with coverage. The dipole moment of the adsorbate remains constant up to Θ?0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near Θ=0.5. The maximum work function increase is Δφmax=0.93 V at Θ=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to Θ=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier which is ascribed to the fact that these quantities may be sensitively influenced by spurious amounts of carbon impurities on the surface.